Nanoscience Workshop by Ranjit T. Koodali

1
Nanoscience WorkshopbyRanjit T. Koodali

• Sinte Gleska University
• July 3 2007

2
Nanoscience Workshop

• Day 2
• Sol-Gel Chemistry
• Nanocrystalline Metal Oxide and
• Solar Energy Conversion

3

Nanocrystalline Metal Oxide Preparation Sol-Gel
Process
Solid materials synthesis procedure, performed in
a liquid and normally at low temperatures Particle
s in a dispersed state in the solvent
independent colloidal suspension Sol The
colloidal particles are linked together to form a
3-dimensional open grid Gel Sol
Gel Gelation process
4

Nanocrystalline Metal Oxide Preparation Sol-Gel
Process
Inorganic gels SiO2, TiO2, ZrO2, Mg(OH)2 etc.
Organic gels -(C-C)- covalent bonds
resorcinol-formaldehyde (RF), melamine
formaldehyde Carbon gels Organic gels heat at
500 oC in N2 ? C gel
5

Nanocrystalline Metal Oxide Preparation Sol-Gel
Process
The usual molecular precursors are
metallo-organic compounds such as alkoxides
M(OR)n, where M is a metal like Si, Ti, etc. R is
an alkyl group (R CH3, C2H5, etc.). For
example, tetraethylorthosilicate (TEOS),
Si(OC2H5)4, is commonly used in the solgel
synthesis of silica and glasses. Similarly
Ti(iOC3H7)4 is used for the preparation of TiO2.
Ti(iOC3H7)4 4 H2O TiO2 4 C3H7OH
Ethanol/Methanol, Catalyst Ti(iOC3H7)4 x
H2O Ti(iOC3H7)4 x(OH)x x
C3H7OH Titanol Ti(iOC3H7)4 x(OH)x
Ti(iOC3H7)4 x(OH)x
Ti(iOC3H7)4 x(OH)x -1OTi(OH)x-1(OC3H7)4-x
H2O
6
Metal Salt (MXn) as precursorHydrolysis reaction
Mz H2O ? M(OH2)z
H2O groups are replaced by OH
(loss of proton)
7
Metal Salt (MXn) as precursor Condensation
reaction
H2O groups are eliminated
8
Hydrolysis Mechanism
9

Nanocrystalline Metal Oxide Preparation Sol-Gel
Process
Ti(iOC3H7)4 4 H2O TiO2 4
C3H7OH Hydrolysis (Initiation)
Ethanol/Methanol, Catalyst Ti(iOC3H7)4 x H2O
Ti(iOC3H7)4 x(OH)x x
C3H7OH Titanol Condensation
(Propagation) Ti(iOC3H7)4 x(OH)x Ti(iOC3H7)4
x(OH)x Ti(iOC3H7)4 x(OH)x
-1OTi(OH)x-1(OC3H7)4-x H2O
10

Nanocrystalline Metal Oxide Preparation Sol-Gel
Process

• Two types of gels depending on the rate of
  gelation process
• Colloidal Gels hydrolysis rate is slow and
  condensation rate is slow
• Polymeric Gels (no formation of intermediate
  occurrence of individual particles) - hydrolysis
  rate is fast and condensation rate is slow
• Controlled Precipitation - hydrolysis rate is
  slow and condensation reactions are fast
• Gelatinous Precipitation both hydrolysis and
  condensation reactions are fast

11

Solar Energy?

• World demand for energy is projected to more than
  double by 2050 and to more than triple by the end
  of the century.
• More energy from sunlight strikes the Earth in
  one hour (4.3 1020 J) than all the energy
  consumed on the planet in a year (4.1 1020 J).
• In 2001, solar electricity provided less than
  0.1 of the world's electricity, and solar fuel
  from modern (sustainable) biomass provided less
  than 1.5 of the world's energy.
• Solar electricity from photovoltaics is too
  costly, by factors of 510, to compete with
  fossil-derived electricity.
• Solar energy has enormous promise as a clean,
  abundant, economical energy source.
• Research challenges exist in designing materials
  and in understanding the electronic and molecular
  basis of capture, conversion, and storage before
  its
• promise can be realized.

Report on the Basic Energy Sciences Workshop on
Solar Energy Utilization April 18-21, 2005,
Bethesda, Maryland
12

Automobiles were disruptive to the horse and
buggy business. PCs were disruptive to the
typewriter industry. We believe solar electric
systems will be disruptive to the energy
industry. Daniel Shugar, PowerLight Corporation,
in National Geographic, August 2005
Photovoltaics
Crystalline Silicon Solar Cells Advantages
Raw material is abundant, durable,
environmentally benign 22 efficiency
(incoming light to electricity)Disadvantages
Producing high purity silicon is expensive Low
absorptivity (100 µm thick is heavy and rigid)
High demand for computer chips raises price
(100/kg) 3-5/watt
http//www.mrsec.wisc.edu/Edetc/SlideShow/slides/p
n_junction/crystalline.html
13

TiO2 Based Dye Sensitized Solar Cell
http//www.mrsec.wisc.edu/Edetc/SlideShow/slides/T
iO2/gratzel_E.html
14

TiO2 Dye Sensitized Solar Cell
15

TiO2 Dye Sensitized Solar Cell
16

TiO2 Dye Sensitized Solar Cell

• Glossary of Terms
• Absorption (usually of light) One of the possible
  fates of light when it interacts with matter
• Adsorption Binding of a substance to the
  surface of a material.
• Charge Measure of strength of a body by which
  it can interact electrostatically with another
  body
• Chlorophyll Dye/Pigment that is used by living
  plants to convert light energy into chemicals.
• Colloid Small particles (
• Colloidal suspension Collection of small
  particles in a liquid, e.g. milk
• Counter Electrode The electrode at which the
  electrochemical circuit is completed
• Current Rate of supply of charge (Amperes, A)
• Diffusion Flow of molecules from one part(high
  concentration to another (i.e. low
  concentration)
• Dye Chemical compounds that can absorb light
• Electrode A solid that is in contact with an
  electrolyte in a electrochemical cell so that
  current can flow
• Electrolyte An ionically conducting medium
• Electron Transfer Movement of electrons from one
  substance to another
• Energy Capacity to do work
• Iodide Element present in sea water (functions
  as mediator in DSSC)
• Ion Electrically charged atom or groups of atom

17
Glossary of Terms 17. Mediator Molecule that
maintains, and protects the electrical charge on
another substance by being reduced or
oxidized 18.Nano One Billionth of a
meter 19.Nanocrystalline A material made up of
tiny regions in the range of 1-100
nm. 20.Oxidation A reaction in which an atom, an
ion or a molecule loses an electron 21.Photoelectr
ochemical Cell A device in which light driven
reaction induces electron to travel from one
substance to another 22.Photon Packet of
electromagnetic radiation 23.Photosynthesis Proce
ss by which plants use the energy from light to
synthesize compounds for their growth from
carbon dioxide and water 24.Photovoltaic Cell A
device that converts light energy into
electricity 25.Reduction A reaction in which an
atom/ion or a molecule gains an
electron 26.Semiconductor A solid that normally
does not conduct electricity, its electrical
resistance decreases as temperature is
raised 27. Sensitization The process by which a
transparent substance is made to absorb light 28.
Solar Cell A device that can convert light
energy into electrical/chemical energy 29.
Suspension A dispersion of small particles on a
macroscopic scale 30. Tin Dioxide An oxide
material of Tin (Sn) that is electrically
conductive and optically transparent 31.Titaniu
m Dioxide An oxide material of Ti metal that is
used as a white pigment in paint, salads, lip
sticks, etc. 32.Voltage A measure of the
difference in electrical potential energy between
two electrodes or points.

TiO2 Dye Sensitized Solar Cell
18
TiO2 Dye Sensitized Solar Cell
(http//www.mrsec.wisc.edu/Edetc/nanolab/TiO2/inde
x.html)

• Preparing the TiO2 suspension.
• Deposition of the TiO2 film.
• Staining the TiO2 with Dye.
• Carbon coating the counter electrode
• Assembling the Solar Cell Device
• Measuring the Voltage Output

19
TiO2 Dye Sensitized Solar Cell
(http//www.mrsec.wisc.edu/Edetc/nanolab/TiO2/inde
x.html)

• Preparing the TiO2 suspension.

Grind about 2-3 grams of nanocrystalline titanium
dioxide (TiO2) in a mortar and pestle with a few
drops of very dilute acetic acid. Alternate
grinding and addition of a few drops of very
dilute acetic acid until you obtain a colloidal
suspension with a smooth consistency, somewhat
like latex paint. (Very dilute acetic acid is
prepared by adding 0.1 mL concentrated acetic
acid to 50 mL of water.) One batch will use TiO2
sample labeled Degussa P25, another batch will
use TiO2 sample labeled Acros and another batch
will use TiO2 sample labeled Alfa.
Add a few drops of Triton X-100 surfactant or
clear dishwashing detergent and mix some more.
20
TiO2 Dye Sensitized Solar Cell
(http//www.mrsec.wisc.edu/Edetc/nanolab/TiO2/inde
x.html)

• 2. Deposition of the TiO2 film.

Identify the conducting side of a tin
oxide-coated piece of glass by using a multimeter
to measure resistance. The conducting side will
have a resistance of 20-30 ohms.
With the conducting side up, tape the glass on
three sides using one thickness of tape. Wipe off
any fingerprints or oils using a tissue wet with
ethanol.
Add some of the titanium dioxide suspension and
quickly spread using a glass rod. The tape serves
as a 40-50 micrometer spacer to control the
thickness of the titanium dioxide layer. (If the
layer dries out, add more water.)
21
TiO2 Dye Sensitized Solar Cell
(http//www.mrsec.wisc.edu/Edetc/nanolab/TiO2/inde
x.html)

• 2. Deposition of the TiO2 film.

Carefully remove the tape without scratching the
TiO2 coating
Heat the glass on a hotplate in a hood for 10-20
minutes. The surface turns brown as the organic
solvent and surfactant dries and burns off to
produce a white or green titanium dioxide
coating.(Note this requires a plate that gets
quite hot.) Allow the glass to slowly cool by
turning off the hotplate.
22
TiO2 Dye Sensitized Solar Cell
(http//www.mrsec.wisc.edu/Edetc/nanolab/TiO2/inde
x.html)

• 3. Staining the TiO2 with Dye.

Immerse the coating in a source of anthocyanins,
such as raspberry juice. The raspberry juice may
be obtained most easily from frozen raspberries.
(Blackberries, pomegranate seeds, and Bing
cherries can also be used.) The white TiO2 will
change color as the dye is absorbed and complexed
to the Ti(IV).
Rinse gently with water and then with ethanol.
(The ethanol serves to remove water from the
porous TiO2.)
23
TiO2 Dye Sensitized Solar Cell
(http//www.mrsec.wisc.edu/Edetc/nanolab/TiO2/inde
x.html)

• 4. Carbon coating the counter electrode

Pass a second piece of tin oxide glass,
conducting side down, through a candle flame to
coat the conducting side with carbon
(soot). Alternately, use a graphite pencil to
coat the glass piece. For best results, pass the
glass piece quickly and repeatedly through the
middle part of the flame.
Wipe off the carbon along the perimeter of three
sides of the carbon-coated glass plate using a
cotton swab.
24
TiO2 Dye Sensitized Solar Cell
(http//www.mrsec.wisc.edu/Edetc/nanolab/TiO2/inde
x.html)

• 5. Assembling the Solar Cell Device

Assemble the two glass plates with coated sides
together, but offset so that uncoated glass
extends beyond the sandwich. Do not rub or slide
the plates. Clamp the plates together.
Add a few drops of a triiodide solution to the
edge of the plate. Capillary action will cause
the KI3 solution to travel between the two
plates. (The KI3 electrolyte solution consists
of 0.5 M KI and 0.05 M I2 in anhydrous ethylene
glycol.)
25
TiO2 Dye Sensitized Solar Cell
(http//www.mrsec.wisc.edu/Edetc/nanolab/TiO2/inde
x.html)

• 6. Measuring the Voltage Output

Connect a multimeter using an alligator clip to
each plate (the negative electrode is the TiO2
coated glass and the positive electrode is the
carbon coated glass).
Test the current and voltage produced by solar
illumination, or test the current and voltage
produced by illumination from an overhead
projector.
26
TiO2 Dye Sensitized Solar Cell
(http//www.mrsec.wisc.edu/Edetc/nanolab/TiO2/inde
x.html)

• Observations
• Nanocrystalline TiO2 Cell Output (V)
• Group A (Degussa) 0.38 V
• Group B (Degussa) 0.43 V
• Group C (Degussa) 0.40 V

27
TiO2 Dye Sensitized Solar Cell
(http//www.mrsec.wisc.edu/Edetc/nanolab/TiO2/inde
x.html)

• Questions
• Write the reactions taking place in the
  nanocrystalline Dye sensitized solar cell.
• Name the semiconductor that is used in the DSSC.
• What is the role of Tin Oxide in DSSC?
• What is the electrolyte solution? Which is the
  reduced mediator and which is the oxidized
  mediator?

28
TiO2 Dye Sensitized Solar Cell
(http//www.mrsec.wisc.edu/Edetc/nanolab/TiO2/inde
x.html)

• Questions
• 6. What is the net reaction in the
  nanocrystalline Dye sensitized solar cell?
• What would you do to increase the voltage output?
• Assuming that a typical home needs 1000 W of
  electrical power in an hour, and assuming that
  solar illumination contains 1000 W/m2 and a solar
  cell is 10 efficient, how large should the solar
  cell or panel be constructed?
• How can we further improve the design of the
  DSSC?